Electrophotography



March 21, 1961 A. ROSE ELECTROPHOTOGRAPHY Filed Oct. 24, 1958 z mu:

INVENTOR. ALBERT RnsE ATTORNEY Uni e s a t n ELECTROPHOTOGRAPHY Albert Rose, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 24, 1958, Ser. No. 769,502

7 Claims. (Cl. 96-1) This invention relates to electrophotography and more particularly to improved methods and means for producing visible powder images in substantial configuration with a latent conductivity pattern in a photoconducting layer.

.In one type of electrophotographic process, a light image is momentarily projected incident upon an uncharged photoconducting insulating layer producing a latent conductivity pattern therein. During the decay of the conductivity pattern the photoconducting layer is electrostatically charged by bombardment with charged particles as by the discharge from a corona discharge apparatusa Electrostatic charges build up in the less conducting areas of the photoconducting layer thereby producing a latent electrostatic image in substantial configuration with the conductivity pattern. image is then developed to a visible powder image by any of the well known methods as by contacting the photoconducting insulating layer with an electroscopic powder.

In a second type of electrophotographic process a light jimage is momentarily projected incident upon an uncharged photoconducting layer producing therein a latent conductivity pattern. Then, during the decay of the conduct-ivity pattern, a unidirectional electric field is estab- -lishcd through the photoconductive layer and, with the electric field applied, a dry physical mixture of electrostatically attractable developer powder particles and car- .rier particles is contacted across the surface of the photoconductive layer. u'pon the photoconducting layer in substantial configura- Developer powder particles deposit tion with the conductivity pattern in those areas which have not been exposed to light.

v In a third type of electrophotographic process, a latent conductivity pattern is established in a photoconductive This conductivity pattern is tive backing member underlying the photoconductive layer. Silver ions are thus attracted to the conductive areas of the photoconductive layer where each silver ion gains an electron and deposits on the layer as a silver atom.

All of the foregoing processes have some disadvantages; In the first process, corona discharge is employed requiring potentials of the order of 7,000 volts. In the second process, potentials ranging from 500 to 1500 volts are preferred and optimum results are obtained employing voltages from 700 to 1500 volts. In the third process,

lower voltages are used of theor'der of 25 to 150 volts, however, considerable current, of the order of /2 an The high voltages used in sults in a short operating life for any portable power The latent electrostatic supply. In addition to these disadvantages, the second and third processes require accurate control of develop ing potentials in order to prevent arcing through the photoconductive layer.

An object of this invention is to provide improved methods and apparatus for electrophotographic printing.

Another object is to provide improved methods and apparatus for directly converting a latent conductivity pattern into a visible powder image.

Still another object is to provide improved methods and means for producing a visible powder image in substantial configuration with a latent conductivity pattern which methods employ substantially lower potentials and power consumption than were hitherto deemed possible.

Yet another object is to provide improved methods and apparatus for electrophotography employing a photoconductive layer on an insulating backing member.

-der itself is deposited.

In general, the foregoing objects and other advantages are attained by the process of this invention which include the following steps. First, the recording element, which includes a photoconductive insulating material, is darkadapted by storage for at least one hour in darkness. Then, a latent conductivity pattern is established in the photoconducting material by projecting a radiant image thereon momentarily. Within the decay time of the conductivity pattern so produced a conductive type developer powder is brought into contact with the photoconducting material while simultaneously applying a bias voltage to the developer powder with respect to the opposite side of the material. With a bias voltage of from 50 to 500 volts, the maximum current drawn is of the order of 10 microamperes per square centimeter or less. When a negative bias is applied to the developer powder, the charged powder contacting the unilluminated insulating areas of the photoconducting material adheres thereto. In the illuminated conducting areas of the photoconducting material the charge from the powder but not the pow- If desired, the contrast of the developed image may be varied, the developed image may be reversed and spurious deposit of the powder in background areas controlled by varying the strength and polarity of the bias on the developer powder.

Electrophotographic apparatusin accordance with this invention comprises means for transporting an electrophotographic recording element past an exposure station and a developer station, means at the exposure station for establishing a radiant image incident upon the electrophotographic recording element, means at the developer station for contacting one side of the recording element with a semiconductive or conductive type developer powder and connection means associated therewith for applying a bias voltage to the developer powder with respect to the opposite side of the recording element.

The foregoing objects and other advantages will be more fully described in the following detailed description when read in connection with the accompanying drawings in which:

Figure 1 is a partially sectional, partially schematic view of an apparatus for producing a latent conductivity I pattern in a photoconducting layer.

Figure 2 is a partially sectional, partially schematic view of an apparatus for producing a powder image from the latent conductivity pattern of Figure 1.

Figure 3 is a curve illustrating the changes in conductivity of an incremental area of a photoconducting layer of Figures 1 and 2 during the processing steps illustrated by those figures.

Figure 4 is a partially sectional, partially schematic view of an electrophotographic camera for producing visible powder images from projected light images in accordance with the methods of the present invention.

Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figure 1, a photoconducting layer 23, such as photoconducting zinc oxide dispersed in resinous polysiloxane and supported upon a backing 21 such as paper, is maintained in darkness for several hours prior to the time when it is desired to use it as a recording element. A photographic transparency 24, bearing an image to be copied, is positioned on top of the photoconducting layer 23. An exposure is then made using a light source 25, such as a 40 watt tungsten lamp spaced about from the photographic transparency 24, for about one second. A light image is thus projected on to the photoconducting layer 23 causing an increase in the electrical conductivity of the illuminated areas. This change in conductivity over portions of the photoconducting layer 23 is referred to as a conductivity pattern. The conductivity pattern is latent and is substantially in the same configuration as the incident light image.

Referring to Figure 2, the photographic transparency is then removed from the photoconducting layer and the latent conductivity pattern is developed to a visible powder image by placing the photoconducting layer 23 face down upon the surface of a supply of conductive-type or semiconductive-ty-pe developer powder 28 contained in a metal tray 29. A plate electrode 27 is super-imposed upon the backing member 21. A battery 31, or other source of biasing voltage, is connected to the supply of developer powder 28 through the tray 29 and to the plate electrode 27. The battery 31 is connected to the developer powi der and the electrode plate through a double throw reversing switch 33 and a potentiometer 35 in order that the applied biasing voltage may be changed in polarity and varied in magnitude. Suitable conductive-type developer powders include powdered iron, copper, bronze,

carbon and mixtures of carbon with powdered metal. Semiconductive-type developer powders such as American Process zinc oxide and various phosphors may be employed equally as well. The principal criterion for the developer powders is that they have a volume resistivity not in excess of 10 ohm-cms.

In the developing step of Figure 2, the switch 33 and the potentiometer 35 are adjusted such that about +250 volts is applied to the developer powder with respect to the plate electrode 27. The powder deposits on the areas of the photoconductive surface which were not illuminated. The visible powder image produced by this procedure has a very high contrast characteristic and a minimum amount of spurious deposit in background areas. This set of characteristics is considered ideal for line drawings and line prints. By reducing the biasing voltage, the contrast between the dark and light areas is reduced making it possible to obtain any desired contrast characteristic over a very wide range of contrast values. A preferred range of voltages is about +50 to +500 volts.

If the switch 33 is reversed and an increasing voltage applied to the developer powder 28, a value is reached where there is developed a reverse powder image, that is, the powder deposits in the more conducting areas of the photoconducting layer. The contrast of the reverse visible image increases and the amount of spurious deposit in the background areas decreases as the negative voltage is increased in magnitude. A preferred range of negative voltage is from about 50 to 500 volts.

According to the invention, a direct or reverse visible powder image may be obtained from the same latent conductivity pattern. A direct visible powder image is a developed image wherein the developed areas correspond to the non-illuminated areas of the original light image. A reverse powder image is a developed image wherein the developed areas correspond to the lighted areas of the original light image. By adjustment of the biasing voltage, any desired contrast characteristic over a wide range may be obtained for either the reverse or direct image. Thus one may simply and quickly adjust an electrostatic printing apparatus to produce line prints of high contrast value, continuous tone prints of intermediate contrast value, and in each case, the print may be direct or reverse.

Any photoconducting layer usable in electrophotography may be employed in the invention. Some of these are described by C. J. Young and H. G. Greig in ElectrofaxDirect Electrophotographic Printing on Paper, RCA Review, December 1954, volume 15, No. 4, pages 469 to 484; by E. Wainer in Phosphor Type Photoconducting Coatings for Continuous Tones in Electrostatic Electrophotography, Photographic Engineering, volume 3, No. 1, 1952, pages 12 to 22; and by A. Middleton in U.S. Patent No. 2,663,636 issued December 23, 1953.

Other suitable photoconductive coatings include photoconducting zinc oxide (Florence Green Seal 8 marketed by the New Jersey Zinc Sales Co., New York, N.Y.) dispersed in resinous polysiloxane; photoconducting zinc selenide (Mallinkrodt No. 8856 marketed by the Mallinkrodt Chemical Works, New York, N.Y.) dispersed in resinous polystyrene; photoconducting zinc sulfide (Cryptone ZS 800 marketed by the New Jersey Zinc Co., Palmerton, Pa.); and, panchromatically sensitive zinc oxide prepared according to either U.S. Patent No. 2,727,807 or 2,727,808 to S.M. Thomsen dispersed in resinous polysiloxane. In place of resinous polysiloxane or polystyrene, resinous polyvinyl acetate resinous polyvinyl-chloride-acetate, carnauba wax, guar gum or other electrically insulating, film-forming vehicles may be used.

A preferred composition may be prepared by intimately mixing 100 grams of a photoconducting zinc oxide with 65 grams of a 60% solution of resinous polysiloxane in xylene and grams of toluene.

After ball milling to obtain a smooth uniform consistency, the mixture is coated on the surface of a proper backing member and dried. Any standard coating technique may be used, such as flowing, spraying, dipping, spin coating, or brushing on. Suitable backing members include paper, mica, glass or various cellulosic insulating sheets. In addition to these, relatively conductive backing members may also be employed, however, it is preferred to use an insulator having a volume resistivity of 10 ohm-cms. or higher.

An essential part of the developing procedure is that it must be carried out during the decay of the conductivity pattern in the photoconducting layer. Referring to Fig.

3, there is shown a plot of conductivity in an incremental the conductivity rises to a maximum value as shown by the curve B-C. After the light is turned oif at 2 the conductivity begins to fall to a minimum value during the period 2-3 as shown by the curve CD. It is during the interval 2-3 that development of the latent conductivity pattern must take place. Development is preferably carried out as soon as possible in order to take advantage of larger differences in conductivity. A photoconducting layer such as photoconducting white zinc oxide dispersed in resinous polysiloxane has a very long period of decay and development may take place during a period of at least 16 hours after exposure to the light image. However, development preferably takes place during a period of 30 minutes after exposure to the light image.

Referring to Figure 4 the improved methods and means of the invention may be embodied in an improved electrophotographic camera as illustrated therein. A continuous web comprising a paper back-ing 51 having on one surface thereof a photoconducting insulating coating 53 is unwound from a roll 55.

- I he" -continuous web passes to an exposure station ivhere a light image is projected incident upon the surface of the photoconducting coating 53. For example, employing conventional photographic methods, the real image 57 is projected onto the photoconducting coating 53 by means of a lens 59. In this arrangement, the image incident upon the photoconducting coating 53 will constitute a mirror reversal of the real image 57. If desired, mirror reversal may be eliminated by incorporation in the camera of a double lens structure or a reversing mirror. As a result of the light image impinging'upon the photoconducting layer 53 a latent conductivity pattern is produced therein substantially corresponding to the light image.

The continuous web next advances to a developer station where the latent conductivity pattern is developed by the methods of this invention, by contact with a conductive type developer powder. A powder tray 61 contains 'a supply of conductive type developer powder 63 in contact with the photoconducting coating 53. A sec- 0nd powder tray 65 contains a second supply of conductive-type powder 67 in contact with the back of the continuous web and in opposed relationship to the developer powder 63. The conductive type powder 67 in contact with the back of the continuous web functions as a backing electrode therefor and provides a convenient means for insuring intimate contact with the back of the web during development thereof. Alternatively a conductive bar, plate, or roller in contact with the back of the continuous web may be employed in place of the conductive-type powder 67.

A battery 31, or other source of potential, is connected to the two trays 61 and 65 through the double throw reverse switch 33 and the potentiometer 35. The switch 33 and potentiometer are adjusted to provide the desired biasing voltage for example, a voltage of about +350 volts. -As the continuous web passes between the two powder trays 61 and 65, particles of the developer powder 63 adhere to the non-conducting areas on the photoconducting coating 53 to produce thereon a visible powder image. The powder image so produced is a direct image. Either direct or reverse visible powder images may be produced, spurious deposit of powder in background areas minimized, and contrast value of the visible image varied by adjusting the polarity and magnitude of the bias voltage on the developer powder 63 in a manner like that described in connection with Figure 2.

The continuous web now passes between a pair of pressure rollers 69 and 71. In passing between these rollers developer powder adhering to the photoconducting layer 53 is pressed into the layer and thus fixed thereon. The pressure rollers 69 and 71 are rotated by a conventional crank mechanism, not shown, to provide the means for transporting the continuous web past the exposure and developer stations in the camera.

Although the apparatus above described utilizes pressure rollers to fix the powder image, fixing methods other than the pressure rollers are contemplated within the scope of this invention. For example, the powder image may be fixed to the photoconducting coating by means of a fixative spray such as varnish, shellac or methyl-methacrylate as are commonly employed for the fixing of charcoal drawings. Another fixing method contemplated is to heat thephotoconducting layer by any conventional means to cause the binder material in the layer to soften and the powder image to adhere thereto. It is also possible to soften the binder material with a solvent therefor to cause the powder image to adhere thereto and then evaporate off the solvent. Any of these fixing methods may also be employed in combination with the pressure fixing caused by the rollers 69 and 71 in order to provide more permanent fixed powder images.

There have been described improved methods and means of electrophotography including improved methods and means for developing latent conductivity pat terns. There have also been described methods and means of producing direct or reverse visible images and for controlling the contrast value of the visible powder image in accordance with improved electrophotographic processes.

What is claimed is:

1. A method of producing a visible powder image on an uncharged electrophotographic element comprising a substrate having a coating thereon and in intimate contact therewith of a finely-divided photoconductor dispersed in an electrically insulating vehicle; said method corm prising the steps of maintaining said element in darkness to substantially reduce the conductivity of said coating; then, before applying any electric field to said ele ment, exposing said coating to a light image to produce thereon a latent conductivity pattern in substantial configuration with said light image, thereafter contacting -a surface of said element with a mass of developer powder consisting solely of material having a volume resistivity not in excess of 10 ohm-cm; and, while said powder is in contact with said surface, applying a bias voltage to said powder with respect to the opposite surface of said element to cause said powder to be attracted to and adhere on said element in substantial configuration with said light image.

2. The process of claim 1 wherein said bias voltage is of the order of 50 to 500 volts.

3. The process of claim ZWherein said bias voltage is about 350 volts.

4. The method of claim 1 wherein said substrate comprises insulating material.

5. Electrophotographic apparatus for producing visible images on an uncharged electrophotographic recording element; said apparatus comprising: means for storing said element in darkness; means for exposing said uncharged element to a light image to produce on one surface thereof a latent conductivity pattern; container means for applying to and maintaining in contact with said one surface of said element a mass of developer powder consisting solely of material having a resistivity not in excess of 10 ohm-cm, and electrical connection means for applying a bias voltage to said powder with respect to the opposite surface of said recording element while said developer powder is in contact with said one surface.

6. Electrophotographic apparatus for producing visi- 'ble images on an uncharged electrophotographic recording element in the form of a continuous web; said apparatus comprising a storage compartment for storing said element in darkness; means for transporting said element from said storage compartment part an exposure station and past a developer station; means at said exposure station for establishing a radiation image incident upon said recording element to produce thereon a latent conductivity pattern in configuration with said radiation image; a vessel at said developer station for containing a dry mass of developer powder consisting solely of material having a resistivity not in excess of 10 ohm-cm. and for maintaining said developer powder in contact with one surface of said recording element, said vessel including a first electrode in contact with said developer powder; a second electrode adapted to contact the other surface of said recording element opposite said developer powder; and connection means on said electrodes for applying therebetween a bias potential.

7. Electrophotographic apparatus for producing visible images on an uncharged electrophotographic recording element in the form of a continuous web; said apparatus comprising a storage compartment for storing said element in darkness; means for transporting said element from said storage compartment past an exposure station and past a developer station; means at said exposure station for establishing a radiation image inci- '7 dent upon said recording element to produce thereon a latent conductivity pattern in configuration With said radiation image; a vessel at said developer station for containing :21 dry mass of developer powder consisting solely of material having a resistivity not in excess of 10 ohm-cm. and for maintaining said developer powder in contact with one surface of said recording element, said vessel including a first electrode in contact with said developer powder; a second vessel at said developer station for containing a dry mass of conductive powder 10 and for maintaining said conductive powder in contact with the surface of said recording element opposite to said developer powder, said second vessel including a second electrode in contact with said conductive powder; and connection means on said electrodes for applying therebetween a bias voltage.

References Cited in the file of this patent UNITED STATES PATENTS 2,808,328 Jacob Oct. 1, 1957 OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pages 12-22 -(l952).

Kallman et al.: 1596-1610 (1955) Physical Review, vol. 97, No. 6, pages 

1. A METHOD OF PRODUCING A VISIBLE POWDER IMAGE ON AN UNCHARGED ELECTROPHOTOGRAPHIC ELEMENT COMPRISING A SUBSTRATE HAVING A COATING THEREON AND IN INTIMATE CONTACT THEREWITH OF A FINELY-DIVIDED PHOTOCONDUCTOR DISPERSED IN AN ELECTRICALLY INSULATING VEHICLE, SAID METHOD COMPRISING THE STEPS OF MAINTAINING SAID ELEMENT IN DARKNESS TO SUBSTANTIALLY REDUCE THE CONDUCTIVITY OF SAID COATING, THEN, BEFORE APPLYING ANY ELECTRIC FIELD TO SAID ELEMENT, EXPOSING SAID COATING TO A LIGHT IMAGE TO PRODUCE THEREON A LATENT CONDUCTIVITY PATTERN IN SUBSTANTIAL CONFIGURATION WITH SAID LIGHT IMAGE, THEREAFTER CONTACTING 